Known in the art is an internal combustion engine wherein a post-treatment system of exhaust gas is comprised of a plurality of exhaust purification catalysts arranged in series in engine exhaust passage, a degree of degradation of each catalyst is found, and a degree of degradation of the post-treatment system as a whole is found from the degrees of degradation found for the catalysts (for example, see Japanese Patent Publication (A) No. 2004-76681).
However, in this case, to secure a good exhaust purification action, it is best to make the catalyst with the lowest degree of degradation among these catalysts handle the purification action of the exhaust gas. That is to say, when for example feeding the post treatment fuel when treating the harmful components in the exhaust gas, if making the catalyst with the lowest degree of degradation handle the purification action of the exhaust gas, a good exhaust purification action can be secured and, in addition, the amount of consumption of post treatment fuel can be reduced. However, in the above internal combustion engines, this is not being considered at all.